Metal nanowires have great potential for immediate use in small electronic circuits, sensitive chemical sensors or any application requiring metal filaments, and will likely be required as interconnects in the nanometer-scale electronics of the future, especially those future electronics which are not based on existing silicon technology. However, there are few methods for preparing nanowires that have such technological utility. More particularly, few methods are capable of producing nanowires that are long (i.e., greater than ten microns in length), uniform in diameter, free standing, and metallic. Two of the most successful approaches have been template synthesis and step-edge decoration.
The template synthesis method, which is described in publications by the research groups of M. Moskovits16-18, C. R. Martin8-15, and P. C. Searson21-27, appears to permit the growth of metal nanowires over a wide range of diameters (from nanometer to micron scale) and for a variety of different metals. Template synthesis involves the growth of carbon, metals, or polymers in the void volumes of a nonconductive porous host such as polycarbonate ultrafiltration membranes, porous Al2O3 films, and track-etched mica crystals all of which possess long (microns or longer), dimensionally uniform pores. In general, these pores are oriented perpendicular to the plane of the porous film or membrane. Nanowires are produced by filling these pores with a conductive material. Template synthesis, however, is limited by its reliance on a template. Because all of the templates listed above possess linear, cylindrical (or prismatic) pores, only linear nanowires can be produced. Moreover, nanowires produced by template synthesis are limited to a maximum length that is equal to the thickness of the porous host membrane, which tends to range from 0.1 to 20 microns.
The step edge decoration method, which is described in publications by Himpsel3-4, Kern5-7, Behm2 and others, involves the selective deposition of a metal or other material, such as CaF2, at atomic step edges on a vicinal single crystal surface. Step edge decoration can be controlled to yield continuous “wires” of varied width and interwire spacing. Long nanowires that are many microns in length have been prepared. Because the dimension of the “wire” perpendicular to the vicinal surface has usually been limited to one or at most two atomic layers, it has not been possible to remove these ultra thin metallic ribbons from the surfaces on which they are synthesized. The technological utility of such nanowires is necessarily limited as a result.
In view of the foregoing, it would be desirable to provide methods that facilitate the fabrication of nanowires that are metallic, long (i.e., greater than ten microns in length), uniform in diameter, and removable from the surface on which they are synthesized and, thus, free standing.